A) \[1{{s}^{2}},2{{s}^{2}},2{{p}^{6}},3{{s}^{2}}3{{p}^{6}},4{{s}^{1}}3{{d}^{5}}\]
B) \[1{{s}^{2}},2{{s}^{2}},2{{p}^{6}},3{{s}^{2}}3{{p}^{6}},4{{s}^{2}}3{{d}^{4}}\]
C) \[1{{s}^{2}},2{{s}^{2}},2{{p}^{6}},3{{s}^{2}}3{{p}^{6}},4{{s}^{2}}3{{d}^{6}}\]
D) \[1{{s}^{2}},2{{s}^{2}},2{{p}^{6}},3{{s}^{2}}3{{p}^{6}},4{{s}^{1}}3{{d}^{8}}\]
Correct Answer: A
Solution :
Key Idea Aufbau principle?According to this principle ?In the ground sate of an atom, an electron enters the orbital of lowest energy first and subsequent electrons are filled in the order of increasing energies.? The sequence of increasing energy in the orbital is follow as 1s, 25, 2p, 3s, 3p, 45, 3d, 4p, 5s, 4d, 5p, 6s, 4\[f\], 5d, 6p, 7s Atomic number of Cr is 24 hence, the electronic configuration will be Cr (24) : \[1{{s}^{2}},2{{s}^{2}},2{{p}^{6}},3{{s}^{2}},3{{p}^{6}},4{{s}^{2}}3{{d}^{4}}\] but the actual configuration is Cr (24): \[1{{s}^{2}},2{{s}^{2}},2{{p}^{6}},3{{s}^{2}},3{{p}^{6}},4{{s}^{1}}3{{d}^{5}}\] This is attributed to the fact that half-filled electronic configuration (ie,\[{{d}^{5}}\]) has lower energy. Thus to acquire increased stability one of the 45-electrons goes into 3d-orbital so that 3d-orbital get half-filled in Cr. The extra stability of half filled electronic configuration is due to their symmetrical arrangement and large exchange energy.
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